VxlanMulticastRecipe: Use functools.combinations for ping endpoints

`functools.permutations` generate even reversed endpoints (both AB and
BA). This should be controlled by `BaseEnrtRecipe`'s `ping_bidirect`.

lnst/Recipes/ENRT/VxlanMulticastRecipe.py

@@ -1,5 +1,5 @@
 from collections.abc import Iterator
-from itertools import permutations
+from itertools import combinations
 from socket import AF_INET
 from lnst.Common.IpAddress import ipaddress, interface_addresses
 from lnst.Common.Parameters import IpParam, IPv4NetworkParam
@@ -100,7 +100,7 @@ def generate_test_wide_description(self, config: EnrtConfiguration):
     def generate_ping_endpoints(self, config: EnrtConfiguration) -> Iterator[PingEndpointPair]:
         host1, host2, guest1 = (self.matched.host1, self.matched.host2, self.matched.guest1)
         devs = [host1.vxlan0, host2.vxlan0, guest1.vxlan0]
-        for dev1, dev2 in permutations(devs, 2):
+        for dev1, dev2 in combinations(devs, 2):
             yield from ping_endpoint_pairs(config, (dev1, dev2))
 
     def generate_perf_endpoints(self, config: EnrtConfiguration) -> Iterator[list[EndpointPair[IPEndpoint]]]: